Well, think of it this way: Just about any level you care to name, you've passed it many times without dying, right? (I guess that doesn't apply to Hell levels for most of us. XD But just thinking of the normal levels...) Unless you lose a life there every time, if you happen to die there once in a while, you can think of it as a fluke.
I'm actually encouraged by games where I have several dumb and unnecessary deaths, because I usually get pretty far anyway, and I can say to myself: "Imagine how far I'll get NEXT time, when I avoid all those easily avoidable mistakes."

It sounds cheesy, but positive self-talk is very important for good performance. Instead of mentally swearing or despairing every time you lose a life, celebrate every time you do something right -- like earning a big crystal (or CRISTAL), prancing safely through a cluster of enemies with two pixels to spare, clearing a tough level without dying, etc. Try not to react to mistakes at all, and praise yourself for each success!
Although it can take some willpower, the results are quite worth it.
